This invention relates to a raster scanner, and more particularly, to a raster scanner system which utilizes optical elements for compensating a differential bow of a scan line.
Referring to FIG. 1, a conventional raster scanner system 10 utilizes a light source 12, a collimator 14, pre-polygon optics 16, a multi-faceted rotating polygon mirror 18 as the scanning element, post polygon optics 20 and a photosensitive medium 22. The light source 12, which can be a laser source, produces a light beam 24 and sends it to the rotating polygon mirror 18 through the collimator 14 and the pre-polygon optics 16. The collimator 14 collimates the light beam 24 and the pre-polygon optics 16 focuses the light beam in the sagittal or cross scan plane onto the rotating polygon mirror 18.
The rotating polygon 18 has a plurality of facets 26, each of which is a plane mirror. The facets 26 of the rotating polygon mirror 18 reflect the light beam 24 and also cause the reflected light 24 to revolve about an axis near the reflection point of the facet 26 of the rotating polygon mirror 18. This reflected light beam can be utilized through the post polygon optics 20 to scan a document at the input end of an imaging system as a raster input scanner or can be used to impinge upon a photographic film or a photosensitive medium 22, such as a xerographic drum (photoreceptor), at the output of the imaging system.
The post polygon optics 20 comprises two spherical lenses 28 and 30, which both together are called F-theta lens, and a wobble correction mirror 32. The lens 28 is spherical concave on the side which receives the light beam and planar on the side that the light beam exits the lens and the lens 30 is planar on the side which receives the light beam and spherical convex on the side that the light beam exits the lens. The wobble correction mirror 32 is a concave cylindrical mirror in the cross scan plane.
In real world situations, a raster scanner typically includes a number of lenses and mirrors to accommodate a specific design. Unavoidable imprecision in the shape and/or mounting of these optical elements will inevitably introduce certain anomalies in the quality of the scan line on the photoreceptor. One of such anomalies is a bow.
Bow is an undesirable character of a scan line which does not form a straight line and bows about a central midpoint. An example of a bow is shown by scan line 34 in FIG. 2. Depending on the types of imprecision in the construction of the apparatus, the bow may bend in either direction relative to a bow free line. A bow 34 (FIG. 2) situated downward is called a frown and a bow 36 (FIG. 3) situated upward is called a smile. Typically a bow happens when the center ray of a light beam scanning a lens does not scan along the optical axis of the lens. The farther the center ray of a beam is from the optical axis of the lens, the more the curvature of the bow.
There is yet another undesirable character which is called a differential bow. A differential bow happens in multi-beam raster scanners in which the light beams are nominally off-axis in the cross scan direction. For example, referring to FIG. 4, if the bow of one beam happens to be a smile while the bow of the other beam happens to be a frown, then the two beams create two lines 38 and 39 in which the separation between the two beams varies across the scan. This phenomenon is called a differential bow. It should be noted that a differential bow can have different shapes such as two scan lines with the same shape bow but a different amount of bow on each scan line or two scan lines with a smile on the top scan line and a frown on the bottom scan line. It is desirable to have all the scan lines straight and parallel so that the beam separation across the scan line would be uniform and therefore, the differential bow would be eliminated.